In the telecommunications industry, trouble-shooting of failed networks by sectionalization is well established. The test method commonly employed is known as the loopback method wherein various nodes or points along a network are equipped with remote units which can provide test data received from a test unit back to the test unit, echoing or looping back transmitted test data. Typically, these field units are normally dormant (transparent) on the line and are only activated in response to local manual selection or, alternately, by remotely transmitted unique dress codes. The nodes are successively looped and tested to sectionalize failures. Some past loopback approaches, such as those of U.S. Pat. Nos. 3,943,305 and 4,582,964, have implemented this concept in a manner which has significance only with respect to conventional analog telephone lines. The method of U.S. Pat. No. 3,943,305 utilizes analog alternating current tone pulses which are decoded to generate an enabling device address. U.S. Pat. No. 4,582,964 expands further on this concept, using additional tone signals to control the main central transmitting unit as a means to remotely select the desired test route.
With respect to digital telecommunication networks, the conventional addressing approach has been to send a unique five-bit code (which contains an embedded device address) to the selected loopback device. A loopback unit is located at each node or test point in the network. When a selected loopback unit receives its own unique activation address, it will responsively loopback all data it receives. The fault is isolated to a particular segment of the line when the transmitting test unit receives the looped-back data and performs a comparison. If a particular line segment is determined to be functioning properly, a reset code is sent to disconnect from the line the test unit located at the end of the good line segment. The next line segment is then looped and tested in an identical manner. This process is repeated until the fault is isolated.
There are several difficulties with the current approach. Most significantly, the utilization of the present industry standard five-bit code imposes severe restrictions on the number of loopback units or other devices which may be employed on a given segment of a network. To optimize testability, every node in the network should provide remote loopback. This requires the generation of a unique device address for each loopback unit located within the network. A five-bit address is limited to addressing a maximum of 2.sup.5, or 32 unique addresses. However, due to certain industry usage restrictions, the industry standard five-bit code is in practice only capable of providing six unique codes for remote loopback device addresses. While the format for sending the five-bit code is well defined throughout the industry, only a few of the available codes are standardized with regard to conveying a specific meaning. The limited number of nodes which can be addressed by a five-bit code is insufficient with respect to most telecommunication networks. In actual practice, the present approach is only capable of testing from one end of a line to the other end of a line, that is, it is incapable of looping at intermediate points along the line. Furthermore, the applicant is unaware of any similar system which has the capability of looping at such intermediate points along the line.
Second, it is imperative to accurately and reliably distinguish between transmitted test data and actual live customer data, even though line tests are typically performed with the line out of service. This is essential in order to prevent inadvertent activation of a dormant test device by live customer data. A more secure and reliable method of addressing dormant loopback or other devices on line would be highly desirable.
Lastly, if a particular line segment is found to be defective, there is often no remote means available to deactivate (reset) a locked-up loopback unit, that is, if the line is intermittently bad, a given loopback unit may receive its activation address code and initiate loopback but may not be able to receive its deactivation reset code. In such conditions, aside from a manual field reset, the line will remain in loopback. Therefore, it would be highly desirable if the loopback were performed simultaneously from both the near and far sides of the line with respect to a given loopback unit. This would enable a given loopback unit to be remotely deactivated (reset), if necessary, from the remaining side of the line that is presumably functional. Additionally, with such a simultaneous near and far side loopback, two line segments could be tested at the same time.
It is, accordingly, the object of the present invention to overcome such prior art deficiencies as stated above and to provide an improved digital telecommunications network test system.